Microwaveable food container with perforated lid

ABSTRACT

A container for use in connection with the exposure of edible matter to microwave radiation. The container comprises a base tray and a lid. The lid is formed from a sheet of material which absorbs microwave radiation and is heated by it, and has a plurality of openings formed in it to allow escape of moisture during heating of the product. The lid is so folded or constructed that the distance through the openings from one side to the other side of the lid is greater then the thickness of the material of the lid.

This invention relates to a container for use in connection with theexposure of a product to microwave radiation, and to a method ofexposing edible matter to microwave radiation.

A product which is heated by microwave radiation is heated effectivelyfrom within the body of the product. The product is exposed to air whichis cool and humid, compared with that to which a product might beexposed to in a conventional oven. Moreover, the period in which aproduct is exposed to microwave radiation in a microwave oven isgenerally very much less than the period of exposure to heat in aconventional oven. As a result, it is often found that it is notpossible for adequate moisture to be lost from a product which is heatedby exposure to microwave radiation so that, in the case of certainproducts, in particular edible products which are required to have acrisp external surface layer, the surface layer can be soggy andgenerally devoid of crispness.

A number of packaging materials have been developed for use withmicrowave appliances in order to alleviate this problem. Such packagingmaterials include several materials capable of absorbing microwaveenergy, and converting that energy to heat. They are sometimes referredto as "microwave heat-reactive materials". The food product in questionis covered, preferably enveloped, by the packaging material, and thensears due to exposure to the heat generated within the material by themicrowave radiation.

A known such packaging material is disclosed in EP-A-270838. Itcomprises a plastic film having a thin metallic coating applied to it.The film has a plurality of slits in the form of broken lines, inparallel, formed in it. The slits allow the material to be deformed soas to fit around a food product. Deformation of the material causes theslits to open. This known material has been found in certaincircumstances to have the disadvantage that it can be difficult for aperson to position the material around a food product satisfactorily toensure that the product within the material is heated in a controlledmanner, such that the exposure of the food product to the radiation iswithin desired limits, and that the characteristics of the heated foodproduct are optimised.

According to the present invention, it has been found that a foodproduct can be exposed to microwave radiation so that the inner regionsof the product are suitably heated, and that the surfaces of the productare dried and made crisp, by placing the product in a tray with a lidhaving a plurality of openings extending through it, and being so foldedthat the distance through the openings from one side to the other sideof the lid is greater than the thickness of the lid material.

Accordingly, in one aspect, the invention provides a container for usein connection with the exposure of edible matter to microwave radiation,which comprises a base tray and a lid, the lid being formed from a sheetof a material which absorbs microwave radiation and is heated by it, thelid having a plurality of openings extending through it, and being sofolded or constructed that the distance through the openings from oneside to the other side of the lid is greater than the thickness of thematerial of the lid.

The container of the invention has the advantage that the externalsurface of a product can be exposed directly to hot dry air, which has alow relative humidity, so that evaporation of water from the surface ofthe product is facilitated. Water vapour which results from evaporationof moisture on the surface of a product exposed to microwave radiationcan then be removed from the product through the openings in the lid sothat the surface of the product can acquire a low surface moisturecontent, and therefore also to be crisp.

Furthermore, the presence of the base tray as a component of thecontainer allows a food product to be packaged in the container in whichit is to be prepared for consumption, so that it can be passed from theoriginal manufacturer, through the relevant retail chain, to the personwho is to heat the product, and then be heated without having to beremoved from its original container. This represents a significantadvantage since handling of the food product is minimised duringtransportation and prior to heating, so that the likelihood of damage tothe product prior to heating is reduced. This can be particularlyadvantageous when the food product is fragile at least prior to heating,which can be the case with many pastry products, with which thecontainer of the invention is particularly well suited for use.

A further significant advantage of packaging a food product in acontainer in which it can be heated is that the heating of the productby the consumer is made easier; it is necessary for the consumer only toremove any outer wrapping from the container and to fix the lid over thebase (if this has not been done already), prior to heating the productin a microwave oven. This is entirely consistent with the desire toprovide easily prepared meals quickly, with minimum handling andexpenditure of time.

Preferably, the distance through the openings from one side to the otherside of the lid is at least about 5 times greater than the thickness ofthe material of the lid, more preferably at least about 10 timesgreater. In this way, the function of the openings becomes similar tothat of an array of chimneys. Convection currents generated within thecontainer can facilitate removal of moisture from the surface of thefood product, and the convection currents are enhanced by the use of amaterial for the lid of the container which is folded to providerelatively long chimneys.

The openings in the lid may be provided as slits. The slits willgenerally extend approximately parallel to one another. Preferably, theslits are arranged in parallel lines, with non-broken film between pairsof slits in each line. It is preferred that slits in a first lineoverlap with non-broken film located between respective pairs of slitsin an adjacent second line. Openings provided in a lid in the form ofslits with these preferred features can be opened so that the distancethrough the openings from one side to the other side of the lid isgreater than the thickness of the film, simply by pulling the lid in adirection substantially perpendicular to the lines of the slits.

The slits in the lid of the container may have an arcuate configuration.For example, the slits may be arcuate in the sense that they are definedby a constant radius along their length. Other configurations of curvedslits are envisaged. It can be preferred for the configuration of thearcuate slits to be varied over the surface of the lid. For example, thedegree of curvature of the slits may vary from one slit to another, orfrom one region of the lid to another.

The provision of slits in the lid having an arcuate configuration hasthe advantage that it allows the lid to conform to the shape of the foodproduct more closely. A further advantage is that it has been found thatthe lid has a reduced tendency to flare outwardly at its ends.

The lid may have a plurality of substantially parallel lines of weaknessextending across it along which the lid is or can be folded, andopenings extending through the lid along at least every alternate lineof weakness. A lid with openings having this configuration can have theadvantage that the space under the lid for hot air, between the lid andthe upper surface of the food product, is greater than in the case of asheet in which openings are formed by stretching a sheet which remainssubstantially in one plane.

A further advantage of a lid with openings along spaced apart lines ofweakness is that the openings in the lid are formed during manufactureof the lid, so that the configuration of the openings can be controlled.This can be contrasted with susceptor products in which openings areformed by stretching when they are wrapped around a food product by aconsumer immediately prior to exposure to radiation.

Preferably, the openings are spaced apart substantially regularly alongthe lines of weakness.

The size of the openings in the lid will be selected according tofactors such as the amount of moisture which it is desired to allow tobe lost from the surface of a food product in the container. It has beenfound appropriate for the ratio of the transverse dimension of theopenings to the distance between adjacent lines of weakness has a valueless than about 1.5, preferably less than about 1.0. Preferably, thevalue of the ratio is at least about 0.5, especially at least about 0.8.Generally, the openings will be centred on the apices of thecorrugations in the lid.

Preferably, the openings in the film are generally rounded, for exampleapproximately circular.

Openings in the lid may be defined by at least three flaps arrangedaround the opening, and the flaps being folded out of the plane of thelid.

The lid is formed from a material which absorbs microwave radiation andis heated by it. Such materials are sometimes referred to as "susceptormaterials". For example, the lid may comprise a layer of a metal, forexample in a metallised polymeric film. The metal may be applied byvacuum deposition, for example of a metal such as aluminium. A suitablematerial is disclosed in GB-A-2046060.

The lid may comprise other microwave absorbent materials, such ascertain ferromagnetic materials, and materials which have a carbon basedcoating.

When the lid comprises a metallised polymeric film, the polymer ispreferably a polyester. The film may include a laminated layer of paper.The paper may serve as a support for the film, and may be a stiffenedpaper, for example a card, in order to provide the desired degree ofsupport.

It can be preferred in many circumstances for both the lid and the baseof the container to be made from one or more susceptor materials, whichabsorb microwave radiation and are heated by it.

Preferably, the lid is attached to the base of the container along atleast part of one edge of the base. The lid and the base may be formedas a single article, being joined along a fold line.

It is preferred that the lid and the base of the container are providedwith means by which the lid can engage the base to close the container.For example, the lid and the base can be provided with cooperatingformations, such as tabs and slots, or wrap over portion on the lid toengage a rim on the base.

It can be convenient for the base of the container to be formed from thesame material as that of the lid, particularly when the lid and the baseare joined to one another. The formation of the base from a susceptormaterial has been found to enhance the heating effect of the microwaveradiation on a food product within the container, while the lid canprovide the venting which can give rise to a crisp product. Indeed,generally it has been found to be possible for the base not to have anyopenings in its walls, and for the characteristics of a food productheated in container not to be affected adversely. This has the advantagethat fluid which seeps from the food product while it is being heatedcan be collected in the container, without dirtying the interior of themicrowave oven or any need to use a secondary container.

The base of the container may be formed by folding a sheet of a suitablematerial. The container may be retained in its folded configuration byuse of a system of interlocking formations, for example in the form oftabs and slits. Alternatively the container may be retained in itsfolded configuration by use of an adhesive.

The base of the container may be formed by moulding, for example, byinjection moulding. This has the advantage of providing by a simpletechnique a base which can collect fluid which seeps from a food productas it is heated.

In another aspect, the invention provides a method of exposing ediblematter to microwave radiation, which includes the step of placing thematter within a container of the type discussed above. The method mayinclude the step of stretching the lid of the container to causeopenings in it to open.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a plan view of a cut sheet of card, from which a container canbe formed;

FIG. 2 is an isometric view of a container formed from the sheet shownin FIG. 1, before its lid is closed;

FIG. 3 is an isometric view of the container shown in FIG. 2, with thelid closed;

FIGS. 4a and 4b are plan and isometric views respectively of anotherembodiment of container; and

FIG. 5 is an isometric view of a further embodiment of container.

Referring to the drawings, FIG. 1 shows a cut blank formed from a sheetof susceptor material, which can be folded to form a container. Thematerial comprises a metallised polyester film which has been laminatedto a sheet of card, the metal surface of the film being in contact withthe card.

The blank comprises a base portion 2, end walls 4, 6, and front and backwalls 8, 10 respectively.

A lid 12 is connected to the back wall 10. The lid is formed with aplurality of slits 14 arranged in parallel lines 16. Non broken lidmaterial exists between pairs of slits in each line. Slits in a firstline overlap with non-broken lid material located between respectivepairs of slits in an adjacent second line. This arrangement of slitsallows openings to be formed in the lid by stretching the lid in adirection substantially perpendicular to the lines 16 of slits.

An edge portion 18 of the lid 12 is provided with a pair of tabs 20. Thetabs are sized and located so as to engage recesses 22 in the front wall8 of the base 2.

Each of the front and back walls 8, 10 has tabs 24, which can engageappropriate slits 26 in the end walls 4, 6, when the base is assembled.

FIG. 2 shows the blank shown in FIG. 1 after it has been folded, andafter the tabs 24 have been engaged with the slits 26, to lock the baseinto its folded configuration. The container is arranged with thepolymer surface of the sheet disposed internally, and the paper surfaceof the sheet disposed externally.

A food product 28 can be placed in the container once it has beenassembled, to have the configuration shown in FIG. 2. The lid can beretained below the base, by folding the lid away from the upper surfaceof the container. The food product can be packaged in the container bymeans of an external wrap. The wrap can be transparent, at least fromabove, so that the food product can be inspected by a consumer prior topurchase.

Before heating food product using the container shown in FIG. 2, anyouter wrapping material is first removed. The lid is then drawn over thebase to cover the food product. As it is drawn over the base, the slitsin the lid are opened, and a 3-dimensional array of openings is formed.The openings can be considered to be chimneys, the distance through theopenings from one side of the lid to the other side thereof being atleast five times greater than the thickness of the material of the lid.The lid is fastened over the base by locating the tabs 20 in therecesses 22 on the front wall 8.

The container is shown in FIG. 3, after the lid has been closed.

Exposure of the container, and the food product within it to microwaveradiation causes the food product to be heated. Water vapour from thefood product during the heating process escapes from the containerthrough the openings in the lid, enhanced by convection currents withinthe container.

FIG. 4a shows a cut sheet of card which can be used to form a container,in a manner similar to that in the container of FIGS. 1 to 3. Thepattern of openings in the lid differs from that in the container ofFIGS. 1 to 3. A pattern of slits 32 is formed in the lid 12. The slitsallow the lid to be deformed out of a single plane, by the applicationof force to a point 34 at the centre of the lid. This deformation takesplace before the lid is drawn over the base to cover a food product. Theclosed container is shown in FIG. 4b, without a bulky object within thecontainer, which would cause the lid of the container to be deformedupwardly in its centre, opening the slits.

FIG. 5 shows a container which comprises a moulded base 42, which mightbe formed for example from a polymeric material such as a polyethyleneterephthalate, upstanding walls 43, and a lip or flange 44 extendingaround the upper edge of the walls 43.

The lid of the container is provided in the form of a sheet of ametallised polymeric film. The film has parallel lines of weakness 46extending across it, along which it can be folded to form a corrugatedstructure. The film has a plurality of openings 48 in the form ofcircular perforation. The openings are arranged in parallel lines onalternate ones of the lines of weakness. When the film is folded to forma corrugated structure, the openings are located on the peaks of thecorrugation.

A food product can be supplied to a consumer within the base 42. Thebase may be placed on the lid before the assembly is overwrapped forshipping. Preferably, the overwrapping material is transparent to allowthe food product to be inspected by a prospective purchaser.

Prior to heating, any wrapping material located around the base and lidis removed. The lid is then folded to form a corrugated structure, andplaced on top of the base 42 so as to engage and overlie the lip 44. Thecontainer with the food product is then placed in a microwave oven, andthe food product subjected to microwave radiation, by which it isheated.

The invention is not restricted to the designs of lid shown in thedrawings, and other forms of lid may be used within the definition ofthe invention claimed herein.

We claim:
 1. A container for use in connection with the exposure ofedible matter to microwave radiation, which comprises a base tray andlid, the lid comprising a sheet of a material which absorbs microwaveradiation and is heated by it, the lid having a plurality of openingsextending through it, and being so folded or constructed as to define adistance through the openings from one side to the other side of thelid, the said distance being greater than the thickness of the sheet ofthe material.
 2. A container as claimed in claim 1, in which the saiddistance is at least about 5 times greater than the thickness of thematerial of the lid.
 3. A container as claimed in claim 1, in which theopenings are provided as slits.
 4. A container as claimed in claim 3, inwhich the slits extend approximately parallel to one another.
 5. Acontainer as claimed in claim 4, in which the slits are arranged inparallel lines, with non-broken lid material between pairs of slits ineach line.
 6. A container as claimed in claim 5, in which slits in afirst line overlap with non-broken lid material located betweenrespective pairs of slits in an adjacent second line.
 7. A container asclaimed in claim 1, in which the lid has a plurality of substantiallyparallel lines of weakness extending across it along which the lid is orcan be folded, and openings extending through the lid along at leastevery alternate line of weakness.
 8. A container as claimed in claim 7,in which the openings are spaced apart substantially regularly along thelines of weakness.
 9. A container as claimed in claim 7, in which theratio of the transverse dimension of the openings to the distancebetween adjacent lines of weakness has a value which is less than about1.5.
 10. A container as claimed in claim 1, in which openings in the lidare defined by at least three flaps arranged around the opening, and theflaps being folded out of the plane of the lid.
 11. A container asclaimed in claim 1, in which material of the lid comprises a metallisedpolymeric film.
 12. A container as claimed in claim 11, in which thefilm includes a laminated layer of paper.
 13. A container as claimed inclaim 11, in which the polymer of the film comprises a polyester.
 14. Acontainer as claimed in claim 1, in which the lid is attached to thebase along at least part of one edge of the base.
 15. A container asclaimed in claim 1, in which the base is formed from the same materialas that of the lid.
 16. A container as claimed in claim 1, in which thebase is formed by folding.
 17. A container as claimed in claim 1, inwhich the base is formed by moulding.
 18. A method of exposing ediblematter to microwave radiation, which includes the step of placing thematter within a container which comprises a base tray and a lid, the lidcomprising a sheet of a material which absorbs microwave radiation andis heated by it, the lid having a plurality of openings extendingthrough it, and being so folded or constructed as to define a distancethrough the openings from one side of the lid to the other side of thelid such that said distance is greater than the thickness of the sheetof the material.
 19. A method as claimed in claim 18, which includes thestep of stretching the lid of the container to cause the openings toopen.